The present invention relates to power supplies, and more particularly to power supplies that can operate continuously despite interruptions of the input power source.
Conventional switching power supplies convert input power having a first voltage level and a first format (e.g., AC, DC) to output power having a second voltage level and a second format, where the voltage levels and/or the formats are different. The major components employed in these circuits include input and output capacitors, switches, a temporary energy storage element, and a control circuit that senses attributes of the output (such as voltage and/or current) and regulates the operation of the switches based on that feedback. The switch is not a mechanical switch that commonly comes to mind, but electronic devices (such as a bipolar transistor, MOSFET, and diode), that operate at a high frequency upward in the kilohertz or megahertz range or beyond. The energy storage element is usually an inductor or transformer, but a capacitor may be used as well. One or more large capacitors (electrolytic, ceramic, or tantalum) are put in parallel with the input to smooth out the input voltage level that could otherwise have high ripple due to its relatively long distance from the actual power source (e.g., battery and AC mains) and to maintain input level during brief interruptions of the input power. One or more large capacitors are placed in parallel with the output to prevent it from drooping between cycles of the switch's operation and to minimize the output ripple by attenuating harmonics generated from the switching process. A switching power supply may be an isolated type, in which case the grounds for the input and output of the switching power supply are isolated from one another and can take on different values of potential. A transformer is usually used to achieve the isolation. A switching power supply may also be a non-isolated type, in which case the grounds for the input and output of the switching power supply are coupled together as a common ground. A non-isolated switching power supply may be of an inverting type, in which case the voltage of the supply's output is negative while the voltage of the supply's input is positive, or vice versa. A non-isolated switching power supply may also be of a non-inverting type, in which case the voltages of the supply's input and output have the same orientation (e.g., both are positive, or both are negative). A switching power supply may also be a quasi-isolated type, in which case the grounds for the input and output of the switching power supply are allowed to have different values but the potential difference between the grounds is maintained to a selected value by a keeper circuit. A quasi-isolated switching power supply may be of an inverting type or of a non-inverting type.
An AC-to-DC switching power supply is commonly used in a personal computer to convert the AC input mains to a DC output voltage level of 12V and/or 5V. A DC-to-DC switching power supply is commonly used on computer's motherboard to provide the board with a 3.3V and other supplies from the 12V and 5V output port of the AC-to-DC switching power supply. Also, DC-to-DC switching power supplies are also commonly used in battery-powered portable devices, such as music players, personal data assistants (PDAs), cell phones, and GPS navigation equipment. As batteries are typically stable until their end of life, there is no need to couple a large capacitor in parallel with the input port of a battery-powered switching power supply.